This chapter emphasizes how the concepts of validity and validity evidence are important for clinicians to understand as they prepare to conduct clinical assessments, evaluate the psychometric evidence of measures from the assessment instruments, and interpret measures derived from assessment strategies. The validity of scores on an assessment instrument can never be assumed and must be investigated empirically. Because many psychological constructs are not directly observable, tests of the validity of measures (their construct validity) are also tests of the validity of the constructs they purport to measure. The validation process is ongoing and validity evidence is not a stable characteristic of an instrument or a measure: It can change as theories and constructs evolve and can vary across populations, assessment goals and purposes, and assessment contexts. The process of collecting evidence of convergent and discriminant validity, discriminative validity, content and predictive validity, and consequential and incremental validity, along with other psychometric evidence, is facilitated by using measures of specifically defined constructs. Validity evidence can also differ across different dimensions of a construct, the goals of the assessment, and characteristics of the client and assessment context. The chapter ends with recommendations for interpreting the validity evidence for clinical assessment measures.